External Beam Radiotherapy involves the production of a beam of ionising radiation, usually x-rays or a beam of electrons or other sub-atomic particles, which is then directed towards a cancerous region of the patient. This adversely affects the tumour cells, causing an alleviation of the patient's symptoms. Generally, it is preferred to delimit the radiation beam so that the dose is maximised in the tumour cells and minimised in healthy cells of the patient, as this improves the efficiency of treatment and reduces the side effects suffered by a patient. A variety of methods of doing so have evolved.
One principal component in delimiting the radiation dose is the so-called “multi-leaf collimator” (MLC). This is a collimator which consists of a large number of elongate thin leaves arranged side to side in an array. Each leaf is moveable longitudinally so that its tip can be extended into or withdrawn from the radiation field. The array of leaf tips can thus be positioned so as to define a variable edge to the collimator. All the leaves can be withdrawn to open the radiation field, or all the leaves can be extended so as to close it down. Alternatively, some leaves can be withdrawn and some extended so as to define any desired shape, within operational limits. A multi-leaf collimator usually consists of two banks of such arrays, each bank projecting into the radiation field from opposite sides of the collimator. An example of an MLC is shown in EP-A-0,314,214.
WO 2007/124248 shows an MLC with leaves having ridges along their upper and lower edges, to engage in corresponding formations in a leaf support structure. However, this document teaches that the leaves should be aligned relative to the radiation source, i.e. that the sides of the leaf are aligned with the local direction of the radiation.
WO 2007/003925 also shows an MLC leaf having ridges along its upper and lower edges, to engage in corresponding formations in a leaf support structure, but is silent as to the orientation of the leaf relative to the beam.
One factor in the design of a high-quality MLC is the leakage of radiation through the collimator. One likely area for leakage is between the leaves; there must obviously be some degree of separation between the leaves in order to allow them to slide easily relative to the adjacent leaf, and this small gap could allow for leakage. To alleviate this, MLC leaves and their supporting structures can be designed so that the leaves are held at a small acute angle to the beam direction. This means that from the point of view of the beam, the gap between adjacent leaves is closed.